1. Field of the Invention
The present invention relates to a method for manufacturing a lithium secondary battery (typically a lithium ion battery). In more detail, the present invention relates to a method for manufacturing a positive electrode of the battery.
2. Description of Related Art
A lithium secondary battery including a lithium ion battery is smaller in size, lighter in weight and higher in the energy density than a conventional battery. From these features, the lithium secondary battery is widely used in a commercial field (a power source for personal computers and portable terminals). Further, from the viewpoint of high output density, the lithium secondary battery can be used also as a high-output power source for driving vehicles such as hybrid automobiles (HV).
A lithium secondary battery (typically, a lithium ion battery) like this includes a battery case, an electrode body of a positive electrode and a negative electrode housed in the battery case, and an electrolyte (typically, an electrolyte solution). Each of the positive electrode and negative electrode includes, on a current collector, an electrode mixture layer (specifically, a positive electrode mixture layer and a negative electrode mixture layer) mainly made of an active material capable of reversibly storing and releasing a charge carrier (typically, lithium ions). For example, in a positive electrode of a lithium secondary battery, when a slurry-like (including paste-like, and ink-like) composition (a positive electrode mixture layer-forming slurry) prepared by mixing a positive electrode active material, a high-conductivity material (conductive material) and a binder in an appropriate solvent is coated on a positive electrode current collector, a positive electrode mixture layer is formed.
The conductive material is added to improve the conductivity in a positive electrode mixture layer, and, in a general lithium secondary battery, conductive carbon fine particles (typically, carbon black) are preferably used. However, since the conductive carbon fine particles have a small primary particle size such as several tens nanometers and are strong in cohesion force, it is difficult to uniformly disperse in a solvent. During preparation of a positive electrode mixture layer-forming slurry, by increasing an amount of a solvent used for example, the dispersibility of a carbon powder can be enhanced. However, when an amount of solvent is increased like this, a temperature and/or a time for drying the positive electrode mixture layer-forming slurry become unfavorably high and/or long. As a technology for solving the present problem, Japanese Patent Application Publication No. 10-144302 (JP 10-144302 A) discloses that when a positive electrode mixture layer-forming slurry is prepared, firstly, only a conductive material that is difficult to disperse is dispersed in a solvent with a media disperser (ball mill, for example), followed by mixing and kneading with an active material. According to the method, a positive electrode mixture layer-forming slurry that is free from dispersion irregularity and homogeneous can be prepared.
However, according to the study of the present inventors, when a media disperser such as described in JP No. 10-144302 A is used, since a conductive material, a solvent and a crushing medium (balls in the case of a ball mill, for example) are put together, a particle size distribution of the conductive material after dispersion becomes sharp, and when a positive electrode mixture layer-forming slurry like this is dried, a particle size distribution of the conductive material tended to be eccentrically located. Further, there was a concern that due to a strong shearing action between media, particles are disintegrated to increase the viscosity of a dispersing system or to cause excess dispersion (re-agglomeration of once-dispersed particles) thereof. An increase in the viscosity and the localization of a particle size distribution of the conductive material like this not only deteriorate the work efficiency but also unfavorably induce deterioration of battery performance (an increase in IV resistance, for example). Further, when a media disperser is used, there is a care of wear or contamination of the crushing medium; accordingly, also from the viewpoint of a manufacturing technology, a more excellent method is required.